Photolithography is widely used in semiconductor manufacture and other applications. During photolithography, an exposure energy, such as ultraviolet light, is passed through a mask and onto a target such as a semiconductor wafer. The mask contains opaque and transparent regions which form a primary mask pattern. The exposure energy exposes the mask pattern on a layer of resist deposited on the target. Following exposure, the layer of resist can be developed to form a resist mask. In semiconductor manufacture, such a resist mask can be used for etching, ion implantation and other processes.
One recently developed form of lithography is phase shift lithography. With phase shift lithography the mask can include phase shifters on selected portions of the mask pattern. The phase shifters shift the exposure energy passing therethrough, relative to exposure energy passing through an adjacent area of the mask. The phase shifting initiates a wave canceling which helps to overcome diffraction from the edges of the features. In addition, adjacent dark and bright areas are created on the wafer which clearly delineate the corresponding printed features.
One type of phase shift mask includes attenuated phase shifters. With an attenuated phase shift mask the exposure energy is attenuated (i.e., absorbed) as well as phase shifted. FIG. 1 illustrates an attenuated phase shift mask 10. This type of phase shift mask 10 is also referred to as an "embedded" phase shift mask. The attenuated phase shift mask 10 includes a transparent substrate 11 and a pattern of attenuated phase shifters 12. The phase shifters 12 comprise a thin film configured to absorb a portion of the exposure energy and to transmit the remaining portion. Suitable materials for forming the phase shifters 12 include molybdenum silicide, and mixtures of chrome, nitrogen, and oxygen.
With the attenuated phase shifters 12 the transmitted portion of exposure energy is phase shifted relative to exposure energy directed through clear portions of the substrate 11. Preferably the thin film which forms the attenuated phase shifters 12 is selected to provide from 80-95% attenuation of the exposure energy and a phase shift of .pi. (180.degree.). The phase shifted exposure energy interferes with the non-phase shifted energy increasing the sharpness and resolution of the features defined by the attenuated phase shifters 12. With a positive tone resist the features print as thin lines.
Another recently developed technique in photolithography increases the resolution at the target by changing the manner in which the exposure energy illuminates the mask. In particular, the exposure energy is projected through the mask at an angle that is offset from an optical axis of the lithographic system. This illumination technique is referred to as "off-axis" or "tilted" illumination. Representative off-axis illumination techniques include annular illumination and multi-pole illumination.
FIG. 2 illustrates an off axis illumination lithographic system 13. The system 13 includes an optical axis 14 and an exposure source 15 configured to emit a beam of exposure energy such as UV light. The exposure energy is initially directed along the optical axis 14 of the system 13. However, a screen 16 is positioned adjacent to the source 15 and includes an annular aperture 18. The annular aperture 18 is designed to eliminate the center portion of the beam of exposure energy emitted by the source 15. This blocks the on-axis "0" order unmodulated energy and allows the illumination to come from a left side illumination beam and a right side illumination beam 22 that are off-axis with respect to the optical axis 14. The off-axis illumination beams 20, 22 are focused by a condenser 24 at oblique angles of incident (.theta.) onto a mask 26. The mask 26 is located at the object plane of the lithographic system 13 and includes a transparent substrate 27 and opaque features 28 that form a primary mask pattern. After diffracting through the mask 26, the illumination beams 20, 22 are filtered by a spatial filter 30 and focused by an objective lens 34 onto a target 36. The spatial filter 30 includes an entrance pupil 32 which functions to prevent certain orders of the diffraction pattern from striking the target 36. Image formation now occurs by the interference of two beams being the "0" order and either the "+1" or "-1" diffracted beams.
One shortcoming of the off-axis illumination system 13 is that extensive modifications are required to a conventional lithographic system to provide the off-axis illumination. In addition, because the center of the exposure energy must be blocked, the illumination intensity is decreased. The low illumination intensity requires longer photoresist exposure times and decreases the throughput of the targets. Furthermore, the low illumination intensity can cause non-uniformity in the features imaged on the targets.
One recently developed method for off axis illumination places an additional grating mask between the exposure source and the primary mask. The grating mask includes a grating pattern for generating diffracted light. The light diffracted by the grating pattern provides off axis illumination for the primary mask pattern. Such a method is referred to as ATOM (Advanced Tilted Illumination On Mask). This method is described in the technical article by Kang et al. entitled "A new Method of Tilted Illumination using Grating Mask; ATOM (Advanced Tilted Illumination On Mask)", SPIE Vol. 1927, Optical/Laser Microlithography VI (1993) pg. 226.
FIG. 3 illustrates an ATOM lithographic system 38. The ATOM lithographic system 38 includes an optical axis 14 and exposure source 15 as previously described. However, in the ATOM lithographic system 38, a screen 16A allows all of the exposure energy from the source 15 to impinge on the condenser 24. In addition, the condenser 24 rather than focusing on the primary mask 26, focuses the exposure energy on a grating mask 40.
The grating mask 40 comprises a transparent substrate 43 having a grating pattern of phase shift grooves 41 formed therein. The grating mask 40 is similar in construction to a chromeless phase shifting reticle formed with subtractive phase shifters. Light passing through a phase shift groove 41 is phase shifted relative to light passing through an adjacent full thickness portion of the substrate 43. The phase shift grooves 41 function as a diffraction grating to generate diffracted light having an odd numbered order. The resultant zero order diffraction light interferes destructively and the resultant first order diffraction light interferes constructively. The diffracted light impinging on the primary mask 26 comprises a "tilted" or "off axis" illumination beam. The ATOM lithographic system 38 increases the resolution (R) and the depth of focus (DOF) at the target 36. The resolution (R) and depth of focus (DOF) are a function of the angle of incident light (.theta..sub.i), the pitch of the grating members (P.sub.g), the numerical aperture (NA), and the wavelength (.lambda.) of the exposure energy. These relationships can be expressed by the formulae .theta..sub.i =sin.sup.-1 (.lambda./P.sub.g) and R=.lambda./2NA(1+sin .theta..sub.i /NA)=.lambda./2NA(1+.alpha.) where .alpha.=sin .theta..sub.i /NA.
The present invention is directed to an improved lithographic mask in which a diffraction grating and attenuated phase shifters are incorporated into the same mask. The improved mask provides an increased intensity at the target and eliminates the need to align a separate diffraction grating with the primary mask. In an alternate embodiment, a lithographic system can include a conventional phase shift mask with attenuated phase shifters and a separate diffraction grating mask.